Reducing defects in kiln drying lumber

ABSTRACT

A method of pretreating lumber to be kiln-dried wherein the lumber is first immersed in a cold aqueous solution containing black walnut pellicle extract, rich in hydrolyzable tannins, until the wood has absorbed part of the extract by diffusion. The lumber is subsequently frozen prior to entering the kiln. The presoaking treatment, when combined with prefreezing, has a synergistic effect that reduces shrinkage and collapse of the lumber while also allowing an accelerated drying schedule in the kiln.

[ June 25, 1974 REDUCING DEFECTS IN KILN DRYING LUMBER Inventors: Glenn Adair Cooper, Carterville;

Stanley H. Barham, Johnston City,

both of I11.

Assignee: The United States of America as represented by the Secretary of Agriculture, Washington, DC

Filed: Sept. 7, 1973 Appl. No.: 395,196

US. Cl 34/l3.4, 34/5, 117/117,

117/147 Int. Cl. F261) 7/00 Field of Search 117/116, 147, 149; 34/5,

References Cited UNlTED STATES PATENTS 12/1950 Hoffman 34/5 3,309,778 3/1967 Erickson ..34/5

Primary Examiner-John J. Camby Attorney, Agent, or Firm-M. Howard Silverstein; Max D. Hensley [57] ABSTRACT A method of pretreating lumber to be kiln-dried wherein the lumber is first immersed in a cold aqueous solution containing black walnut pellicle extract, rich in hydrolyzable tannins, until the wood has absorbed part of the extract by diffusion. The lumber is subsequently frozen prior to entering the kiln. The presoaking treatment, when combined with prefreezing, has a synergistic effect that reduces shrinkage and collapse of the lumber while also allowing an accelerated drying schedule in the kiln.

6 Claims, No Drawings REDUCING DEFECTS IN KILN DRYING LUMBER A nonexclusive, irrevocable, royalty-free license in the invention herein described, throughout the world for all purposes of the United States Government, with power to grant sublicenses for such purposes, is hereby granted to the Government of the United States of America.

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates generally to reducing drying defects in kiln-dried wood and accelerating actual drying time in the kiln. More particularly, the invention is directed to a method of reducing kiln-dried lumber defects in some species of wood that do not respond well to prefreezing alone.

2. Description of the Prior Art The process of prefreezing lumber prior to kilndrying is known to reduce drying defects in certain species of wood having a high tannin content. Those benefiting most include redwood, incense cedar, tanoak, black walnut and some eucalypts. White oak, black cherry, and American elm are less responsive. Conversely, some woods with a high extractive content apparently are not benefited by prefreezing. However, generally speaking, woods low in extractive content respond little, or not at all, to prefreezing.

The prefreezing technique itself is patented in US. Pat. Nos. 2,534,714 (Hoffman) which teaches thoroughly freezing the wood to prevent checking and warping and 3,309,778 (Erickson) pertaining to reduced shrinking, honeycombing, and collapse, in lumber that is prefrozen and rapidly kiln-dried.

The disadvantage of the above techniques lies in the fact that numerous species are apparently impervious to the treatment in that they fail to respond. In those species, prefreezing does not reduce drying defects.

It is known that redwood, black walnut, and certain Eucalypts (regnans and delagatensis) contain a group of closely related but unidentified hydrolyzable tannins called ellagitannins. Furthermore, other have documented the fact that black walnut and two other Eucalypts (gigantea and sieberiana) contain the specific tannin juglanin. No evidence was found that juglanin was present in redwood or Eucalyptus regrums and Eucalyptus delagatensis. However, it is our belief that the unidentified hydrolyzable ellagitannins are the common extractive factor linking the woods that respond favorably to prefreezing. A readily available source of these hydrolyzable tannins is the skin, or pellicle, of the black walnut fruit.

Our method is thus distinguished from the prior art by its success with some wood that does not respond favorably to prefreezing and by its use of a black walnut pellicle extract which is absorbed into the lumber before freezing. Our method results in the lumber responding favorably to prefreezing as evidenced by fewer drying defects (or faster kiln-drying times) than if prefrozen without absorption of the extract.

Additionally, we have found that soaking the lumber in a cold aqueous solution containing the black walnut pellicle extract, combined with prefreezing, has a syncergistic defect-reducing effect exceeding the additive effect of the individual treatment steps.

Any methodology used in conjunction with the wood drying art must meet the following criteria: l it must not interfere with normal wood use or processing; 2 it m not be rqs v audifitsamm d atads qp properties. The invention we disclose meets these criteria. In addition, our method is inexpensive and simple enough to be used in small industrial plants, it does not lose its effectiveness, and can be applied as a general predrying procedure that enables faster, more defectfree drying.

SUMMARY OF THE INVENTION The lumber to be kiln-dried is presoaked in a cold aqueous solution containing black walnut pellicle ex- :tract, which is rich in hydrolyzable tannins, until a portion of the extract is absorbed by the lumber. The lumber is then prefrozen according to known methods prior to kiln-drying.

. The solution is made by mechanically removing and then crushing black walnut pellicles in a large vat. Cold water is added and the crushed pellicles are allowed to steep for 3 to 4 days. The solution will take on a deep brown hue. A solution pH of from 6.5 to 5.0 is desirably attained. The pellicles obtained from 1 pound of unskinned black walnuts, for every 2 gallons of water used, is our preferred ratio.

The solids are then screened, or otherwise removed from the solution, and the lumber is immersed in the solution for a sufficient time to allow the extract to be partially absorbed into each board. A 24 hour immersion period has been found satisfactory. Following immersion, the lumber may be partially air dried if desired, but it is not required. The presoaked lumber is now ready for freezing.

Wood species that normally fail to respond to prefreezing alone may be kiln-dried with significantly fewer drying defects, particularly shrinkage, or with an accellerated drying schedule, when prefreezing is accompanied by our methodology.

Accordingly, an object of this invention is a method of reducing drying defects in kiln-dried lumber that is non-responsive to prefreezing alone. Another object of this invention is the provision of a simple and economical cold soak chemical treatment that significantly reduces defects in kiln-dried lumber that is prefrozen. A further object of this invention is a method that aids the conservation of our timber resources by reducing the wastes occasioned by lumber drying defects.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In further illustration of our invention the following examples are given:

EXAMPLE I (Exploratory Test Methods) We chose eastern cottonwood (Populus deltoides Bartr.) for our tests because it is difficult to dry, underutilized, low in extractives content, light colored, and has low or zero reponse to prefreezing. Our method is not limited to any particular species however, and it is believed that many species that to prefreezing alone may be min successfully as a treatment with prefreezing. But to be useful to the forest products industry, water extraction only is far more practical. Furthermore, we needed only the ellagitannins and were not specifically after juglanin. We therefore considered primarily the usefulness of a cold water extraction of the pellicles, which would yield crude tannins containing the ellagitannins. One problem is that the ellagitannins can be partially hydrolyzed in warm water, so aqueous tannin extracts may begin to deposit ellagic acid on standing. (Hillis, W. E., Wood Extraclives and their Significance to the Pulp and Paper Industries. Academic Press, New York, 512 p., il1us., 1962.) To avoid this problem we prepared a cold water (10 C.) extract and used it at temperatures below 20 C. Our presumption was that if this procedure worked we would have sufficient basis for proceeding to more practical industry procedures with larger volumes of lumber. For our purposes, we define a cold water solution as having a temperature between 4 C., and 20 C.

We crushed black walnut pellicles in a beaker, soaked them for 30 minutes, and then filtered the mixture until we had about 1 liter of clear extract. The solution was deep brown and had a pH of 6.5.

Eastern cottonwood, green, with two rings per inch and 3 by 4 inches in cross-section, was cut into wafers one-eighth inch along the grain. Forty-eight adjacent wafers were divided into four treatments so that each treatment contained wafers paired with each others treatment. The four treatments were: 1. soaking in extractives and prefreezing at l F.; 2. soaking in extractives only, 3. prefreezing only, and 4. no treatment.

Samples were soaked in the extractive solution for 71 hours at 4 C., while the other wafers were stored at 40 C. After soaking, the wood was a red brown color similar to black cherry. All wafers were measured radially and tangentially before treatment and after drying at premarked points. The wafers were air-dried in the laboratory for days to about 7 percent moisture content. Volumetric shrinkage was calculated and six comparisons were made.

The average percentage of volumetric shrinkage of the Wafers in each treatment was:

Soaked and prefrozen 7.51 percent Prefrozen only 7.74 percent No treatment 8.26 percent Soaked only 8.45 percent All comparisons were statistically significant with the probability of larger t values less than 0.01.

The change in shrinkage was not great, but in the thin wafers where stresses were low this was expected. The most interesting facts to emerge were: first, soaking in pellicle extractives significantly increased shrinkage; and secondly, soaking plus prefreezing resulted in significantly less shrinkage than any other treatment.

These results indicated there were good possibilities for success in lumber where prefreezing effects on rheology and stress development could play a larger role than in the wafers.

EXAMPLE II This test was patterned after the exploratory test except for the substitution of lumber for wafers. We selected 20 green eastern cottonwood boards from each of three mills in Missouri and two mills in Illinois to get 4 wide variability in the genetic stock and wood characteristics. The flat sawn rough No. 1 Common and better lumber 12 feet long and 6 inches wide was dressed four sides to uniform dimensions of inch by 5 inches by 144 inches. Then each board was cut into four 3-fo0t samples and identified by mill, board, and randomly assigned treatment. The four treatments were the same as in the exploratory study. All samples were weighed and measured at premarked points while green and after kiln-drying.

We prepared a fresh extractive solution from the pellicles of pounds of newly fallen black walnuts from 16 widely scattered trees in southern Illinois. The pellicles were removed mechanically, crushed in a large aluminum vat, and steeped in 250 gallons of cold water. After 4 days the solids were screened from the solution which has a pH of 5.85.

All green boards to be soaked were submerged in the extractive solution for 24 hours and then allowed to partially air-dry to the same moisture content they had prior to soaking. The soaked and unsoaked samples to be frozen were placed in a walk-in freezer at 1 0 F. for 24 hours while wrapped in film to prevent drying. During this freezing period the other samples not in the freezer were similarly protected from drying.

After treatment all samples were stacked in a commercial kiln and weighted down. The wood was dried from between 1 l0 and percent down to 7 percent moisture content with a hot schedule, FPL T12/E7, to induce drying defects. After drying, the samples were remeasured and inspected for defects.

There were slight differences in the response of the samples according to their origin, but the trends were the same. Width, thickness, and volumetric shrinkages were not statistically different, except in one case; but the results paralleled those in the exploratory study (table 1).

Again, the lowest volumetric shrinkage was in the soaked and prefrozen group and the highest in the soaked only treatment. Interestingly, there is little difference among the treatments in the width shrinkage; but the thickness shrinkage of the soaked and prefrozen samples is significantly different. Furthermore, the boards containing collapse and honeycomb followed a similar pattern as the shrinkage. In the soaked and prefrozen boards, 15.3 percent of the samples contained collapse and honeycomb; in the prefrozen only, 18.1 percent of the samples contained collapse and honeycomb; the untreated group contained 22.1 percent; and the soaked only group 23.1 percent. The hot drying schedule was obviously too hot for all the material, but extractive soaking and prefreezing definitely reduced the likelihood of collapse and honeycomb occurring.

These results substantiate the occurrence of synergism in the exploratory work in that the combined use of extractives and prefreezing was beneficial, whereas separately the extractive soak resulted in more shrinkage and prefreezing only had almost no effect. However, the beneficial effects needed to be magnified to have real importance, so a second test of cottonwood lumber was conducted to increase the benefits.

EXAMPLE III It was obvious in the first test on lumber that the extractives did not penetrate the wood as thoroughly as necessary to achieve the desired benefits. Color changes were limited to the surface, and shrinkage differences were small. This was due in part to the green condition of the lumber. With the surface at saturation, penetration of the extractives was shallow. To achieve better penetration in this test, we partially air-dried the lumber before soaking it. This was expected to help, first, because air-drying has been shown to increase permeability, and secondly, because there would be a strong diffusion gradient established on immersion. This test was limited to a comparison of two treatments: 1. the full treatment of partial air-drying, soaking, and prefreezing and 2. no pretreatment.

Eight trees to inches d.b.h. were cut into 6-foot bolts to a top diameter of 8 inches. The bolts were sawed into 1-inch boards, matched into 80 pairs by their position relative to the pith. Thus, each board in a pair had nearly the same ring orientation and came from the same group of annula rings.

The rough green boards were dressed seven-eighths inch thick and ripped to width by pairs. After endcoating, the boards were randomly selected for treatments. The two treatments were the partial air-drying, full soak, and prefreezing; and the controls to be kilndried were without a pretreatment. One board from each pair was assigned each treatment.

The boards to be soaked and prefrozen were airdried to 25 percent moisture content from 142 percent moisture content in 15 days. They were then soaked in the same solution used in Example ll for a period of 24 hours and prefrozen at -l0 F. for another 24 hours. After a second air-drying for 4 days from approximately 90 down to 25 percent moisture content they were kiln-dried to 7 percent moisture content in 52 hours.

The matched control boards at 140 percent moisture content were simultaneously kiln-dried to 7 percent moisture content in 3 /2 days. All boards were weighed, marked, and measured for thickness and width before treatment and after drying (Table 2). The FPL Tl 2/E7 drying schedule was deliberately selected to induce defects in both treatments.

All differences were statistically highly significant. However, in this test the major reduction in shrinkage was in width rather than in thickness as in the first lumber test. Virtually all boards in the fast-dried nopretreatment group were severely collapsed. In the full pretreatment group about 70 percent of the boards had slight collapse, but this could be avoided by decelerating the schedule one more day.

TABLE 2 Lumber shrinkages by treatment-second test of matched sets Shrinkage percentage In this test, as in the first, we kiln-dried the material quickly to induce defects. This was necessary to establish differences because a slow drying schedule would dry everything well and differences by treatment would be masked.

It was also recognized that part of the differences in shrinkage and collapse in this test may have been due to the different drying procedure down to 25 percent moisture content. Some question remained if the differences shown were due to the effect of air-drying followed by kiln-drying rather than just the effect of soaking and prefreezing. A third lumber test was therefore made wherein both the controls and fully treated boards were partially air-dried to about percent moisture content. Then, all differences in subsequent kiln-drying could only be attributed to the treatment differences.

EXAMPLE IV This test was made to directly compare partially airdried, extractive-soaked, and prefrozen eastern cottonwood lumber with lumber that was partially air-dried to the same moisture content. Sixty-eight boards were matched into 34 pairs from 5 trees obtained in the same area as the Example III material. These boards were machined, measured, and weighed just as before; and except for the equal moisture contents preceding kilndrying, all sample preparation was the same as in Example III.

The green material was at about 140 percent moisture content. The full pretreatment consisted of half the boards going into the same extractive solution for 24 hours at a partially air-dried moisture content of about percent. After soaking, the boards were dried to 107 percent and prefrozen at l0 F. for 24 hours. After thawing and at a moisture content of 103 percent, the fully pretreated boards were stacked with their matched controls which had dried to 100 percent moisture content. All boards were kilndried to an aver age of 6 percent moisture content in 88 hours using FPL Schedule T10 E7. This schedule is not as severe as that used in Example Ill.

The entire pretreatment sequence required 10 /2 days after the boards were cut: 2 days of air-drying, 1 day soaking, 3 days air-drying, 1 day of freezing, and 3 /2 days in the dry kiln. During the same period the controls were air-dried and did not dry below 100 percent moisture content because the first 5 days it rained. This was fortunate because it helped keep the controls at the same moisture content as the treated boards. However, 5 good drying days would normally drop the moisture content to around 80 percent.

The differences in volumetric shrinkage were statistically highly significant (Table 3). However, the percentage differences in shrinkage between the treated and control boards was not as great as in Example III where the air-drying was part of the pretreatment but not included in the preparation of the controls. This in- TABLE 3 Lumber shrinkage by treatmentthird test of 34 matched sets Shrinkage percentage Treatment Width Thickness Volumetric Full pretreatment No pretreatment Collapse was greatly reduced by the full pretreatment even though a collapse-inducing drying schedule was used. There was one treated board that had collapse which could not be removed by planing off one thirtysecond inch of thickness, whereas 23 of the controls had one thirty-second inch or more collapse. Ten treated boards and 4 controls had very slight collapse that was removed by planing, and 23 treated boards and 7 control boards had no collapse.

We feel that water soluble extractives of black walnut pellicles can be utilized for pretreating certain species of lumber to respond favorably to prefreezing. Extractives soaking alone is not beneficial in drying, and causes more shrinkage than normally occurs. These tests show that a synergistic relationship exists between the pellicle extractives and prefreezing. The combination of the effects of soaking in extractives and prefreezing exceeds the sum of the two pretreatments taken separately. The combined treatment results in a substantial shrinkage reduction, and greatly reduces the likelihood of collapse when the drying rate is accelerated.

Having thus disclosed our invention, we claim:

1. A method of treating lumber to reduce kiln-drying induced defects comprising the following sequential steps:

a. immersing the lumber to be kiln-dried in a cold aqueous solution containing black walnut pellicle extract until said lumber has absorbed a portion of said extract through diffusion, and

b. freezing the previously immersed lumber prior to kiln-drying.

2. The method of claim 1 wherein said solution has a temperature of from 4 C. to 20 C.

3. The method of claim 1 wherein said lumber is immesed for a period of 24 hours.

4. in a method of treating lumber to reduce kilndrying defects that includes freezing of the lumber prior to kiln-drying, the combination with said freezing of the following preliminary step:

a. immersing the lumber in a cold aqueous solution containing black walnut pellicle extract until said lumber has absorbed a portion of said extract through diffusion.

5. The method of claim 4 wherein said solution has a temperature of from 4 C. to 20 C.

6. The method of claim 4 wherein said lumber is immersed for a period of 24 hours. 

2. The method of claim 1 wherein said solution has a temperature of from 4* C. to 20* C.
 3. The method of claim 1 wherein said lumber is immesed for a period of 24 hours.
 4. In a method of treating lumber to reduce kiln-drying defects that includes freezing of the lumber prior to kiln-drying, the combination with said freezing of the following preliminary step: a. immersing the lumber in a cold aqueous solution containing black walnut pellicle extract until said lumber has absorbed a portion of said extract through diffusion.
 5. The method of claim 4 wherein said solution has a temperature of from 4* C. to 20* C.
 6. The method of claim 4 wherein said lumber is immersed for a period of 24 hours. 